1. Field of the Invention

The present invention generally relates to optical pickup heads compatible with multiple optical recording media, and particularly to an optical pickup head compatible with three optical recording media of different formats.

2. Prior Art

An optical pickup head carries out recording and/or reproducing of information such as video, audio or other data from a recording medium. In such system, a semiconductor laser is used for generating a light beam, and an objective lens is used for converging the light beam and forming a focused spot on the recording medium. The recording density of the recording medium is proportional to the size of the focused spot. In general, the size of the focused spot (S) is proportional to the wavelength (λ), and inversely proportional to the numerical aperture (NA), as expressed by formula (1):
S∝λ/NA   (1)

Therefore, to increase the recording density, the size of the spot being focused on the optical disk must be reduced. To reduce the spot size, as can be inferred from formula (1), the wavelength (λ) of the light beam must be reduced and/or the numerical aperture (NA) of the objective lens must be increased. This has been demonstrated by the ongoing development of optical recording media. For example, the wavelength of read beams for compact disks (CDs) is about 780 nm, the wavelength of read beams for digital versatile disks (DVDs) is about 650 nm, and the wavelength of read beams for high-definition DVDs (HD-DVDs) is about 405 nm. Furthermore, the numerical aperture for CDs is 0.45, the numerical aperture for DVDs is 0.6, and the numerical aperture for HD-DVDs is 0.65-0.8.

On the other hand, coma aberration, which occurs due to a tilting of the optical disk, is associated with the tilt angle of the disk, the refractive index of the disk substrate, the thickness of the disk substrate, and the numerical aperture of the objective lens. To ensure an acceptable level of coma aberration with respect to the tilt of a disk for high-density recording, the thickness of the disk substrate is in general reduced accordingly. For example, CDs have a thickness of 1.2 mm, and DVDs have a thickness of 0.6 mm. Further, the thickness of many HD-DVDs is 0.6 mm or less.

In an apparatus for high-density recording onto or playing from a medium such as an HD-DVD, a primary consideration is the compatibility of the apparatus with existing disks including CDs and DVDs. Conventionally, there are two kinds of optical writing and/or reading systems that are used in multi-compatible home entertainment players. In the first kind of optical writing and/or reading system, an independent optical system is provided therein for each type of disk. That is, generally, the optical writing and/or reading system has at least three light sources and three objective lenses for three disks. This kind of writing and/or reading system needs many optical elements, and is unduly large and costly. In the second kind of writing and/or reading system, there are some common optical elements, for example, a common objective lens. This kind of writing and/or reading system is disclosed in U.S. Pat. No. 6,324,150. This kind of writing and/or reading system reduces the total number of optical elements and simplifies the overall configuration. However, the optical performance of the optical pickup head is limited. In respect of the common objective lens, chromatic aberration occurs because each kind of disk operates according to different wavelengths. Further, spherical aberration occurs because the disks have different thicknesses.

Accordingly, an object of the present invention is to provide a small-sized optical pickup head compatible with three different optical recording media, in which optical aberrations are corrected.

To achieve the above object, an optical pickup head compatible with multiple optical recording media in accordance with the present invention comprises: a first light source for emitting a first light beam with a first wavelength; a second light source for emitting a second light beam with a second wavelength; a third light source for emitting a third light beam with a third wavelength; a prism unit disposed beside the first, second and third light sources, the prism unit including a first portion facing the first light source for passing the first light beam therethrough, a second portion facing the second light source for passing the second light beam therethrough, and a third portion facing the third light source for passing the third light beam therethrough; a reflective multi-surface prism disposed beside the prism unit for redirecting light beams by way of internal reflection; a collimating lens disposed beside the reflective multi-surface prism for collimating light beams; and an objective lens adapted to face any of the optical recording media, for receiving light beams and transmitting the light beams to any of the recording media. The prism unit further includes a first aspherical surface for correcting aberrations of the first light beam caused by non-matching between the first light beam and the collimating and objective lenses, and a second aspherical surface for correcting aberrations of the third light beam caused by non-matching between the third light beam and the collimating and objective lenses.

Referring to FIG. 1, an optical pickup head 100 according to a first embodiment of the present invention is illustrated. The optical pickup head 100 is used in an information recording and/or reproducing device (not shown) compatible with a first optical disk (not shown) having a higher recording density, a second optical disk (not shown) having a lower recording density, and a third optical disk (not shown) having an intermediate recording density between the first and second optical disks. Three light sources 1 a, 1 b and 1 c and three photo detectors 2 a, 2 b and 2 c are respectively used to emit a first, second and third light beams and receive a first, second and third return beams with specific wavelengths according to the first, second and third optical disks. In the present embodiment, the first optical disk is a CD which has a longest wavelength of about 780 nm; the second optical disk is an HD-DVD which has a shortest wavelength of about 405 nm; and the third optical disk is a DVD which has an intermediate wavelength of about 650 nm.

The optical pickup head 100 further comprises three holographic lenses 3 a, 3 b and 3 c, a prism unit 4, a pentagonal prism 5, a collimating lens 6, an optical path changer 7, and an objective lens 9. The light sources 1 a, 1 b and 1 c are juxtaposed with the photo detectors 2 a, 2 b and 2 c respectively. The holographic lenses 3 a, 3 b and 3 c are positioned opposite the respective pairs of a light source and photo detector 1 a, 2 a, 1 b, 2 b, and 1 c, 2 c. The holographic lenses 3 a, 3 b and 3 c are for directly propagating a light beam entering from one side thereof, and deflecting a light beam entering from an opposite side thereof.

Also referring to FIG. 2, the prism unit 4 comprises three prisms 41, 42 and 43. The first and second prisms 41 and 42 are juxtaposed on a same side of the third prism 43, and respectively face the first holographic lens 3 a and the second and third holographic lenses 3 b and 3 c. The pentagonal prism 5 is positioned facing an opposite side of the third prism 43. The collimating lens 6 is positioned facing another side of the pentagonal prism 5, and accords with the wavelength of the second light beams so as to converge the second light beams into parallel light beams. The optical path changer 7 is aslant between the collimating lens 6 and the wavelength selector 8, so as to reflect light beams from the collimating lens 6 to the wavelength selector 8. In the illustrated embodiment, the optical path changer 7 is oriented 45° relative to the collimating lens 6 and the wavelength selector 8. The objective lens 9 has a numerical aperture specified by the second optical disk, which is larger than a numerical aperture specified by the first and third optical disks. The wavelength selector 8 is located beside the objective lens 7, to selectively transmit light beams thereto.

The first prism 41 is essentially a rectangular prism with a first aspherical surface, and includes a first incident surface 410 and a first emergent surface 412. In the illustrated embodiment, the first aspherical surface is provided at the first incident surface 410. In alternative embodiments, the aspherical surface can be provided at the first emergent surface 412 or on the third prism 43. The second and third prisms 42, 43 are composite prisms, and respectively include a second and third right- angle prism 420 and 430, a second and third parallelepiped prism 422 and 432, and a second and third interface 424 and 434. The second and third right- angle prisms 420, 430 and the second and third parallelepiped prisms 422, 432 are respectively interconnected by the second and third interfaces 424, 434. The second right-angle prism 420 includes a second incident surface 4200 facing the second holographic lens 3 b. The second parallelepiped prism 422 includes a third incident surface 4220 facing the third holographic lens 3 c, a third emergent surface 4222, and a third reflective surface 4226. The third reflective surface 4226 interconnects the third incident and emergent surfaces 4220, 4222, and is parallel to the second interface 424. A second aspherical surface is formed on the third incident surface 4220 of the second prism 42. In alternative embodiments, the second aspherical surface can be provided at the third emergent surface 4222 or on the third prism 43. The third right-angle prism 430 of the third prism 43 includes a fourth incident surface 4300 facing the third emergent surface 4222 of the second prism 42. The third parallelepiped prism 432 includes a fifth incident surface 4320 facing the first emergent surface 412 of the first prism 41, a fifth emergent surface 4322, and a fifth reflective surface 4326. The fifth reflective surface 4326 interconnects the fifth incident and emergent surfaces 4320 and 4322, and is parallel to the third interface 434.

The pentagonal prism 5, the collimating lens 6, the optical path changer 7, the wavelength selector 8 and the objective lens 9 are sequentially arranged in a common optical path. The pentagonal prism 5 comprises a sixth incident surface 50, a sixth emergent surface 51, and three sixth reflective surfaces 53, 54 and 55 interconnecting the perpendicular sixth incident and emergent surfaces 50 and 51.

Now referring to FIGS. 3A and 3B, the wavelength selector 8 comprises a central portion 81, a middle portion 82 encircling the central portion 81, and a peripheral portion 83 encircling the middle portion 82. The three portions 81, 82 and 83 have different transmission functions. With regard to the central portion 81, light beams with any wavelength can pass therethrough. With regard to the middle portion 82, light beams with the second and third wavelengths can pass therethrough, and light beams with the first wavelength are blocked. With regard to the peripheral portion 83, only light beams with the second wavelength can pass therethrough, and light beams with the first and third wavelengths are blocked. Therefore, the NA of the objective lens 9 is different when focusing the light beams with different wavelengths.

When recording an information signal on and/or reproducing an information signal from the first optical disk, the first light source 1 a emits a first light beam having the wavelength of about 780 nm. After passing through the first holographic lens 3 a along an original direction thereof, the first light beam enters the first prism 41 through the first incident surface 410. The first light beam is converged by the first incident surface 410, and is output from the first emergent surface 412 of the first prism 41. The first light beam then transmits into the third prism 43 through the fifth incident surface 4320. In the third prism 31, the first light beam is reflected by the fifth reflective surface 4326 and the third interface 434, and is then output from the fifth emergent surface 4322.

After exiting the prism unit 4, the first light beam transmits into the pentagonal prism 5 through the sixth incident surface 50, and exits the pentagonal prism through the sixth emergent surface 51 after being reflected by and between the sixth reflective surfaces 53 and 52. The first light beam passes through the collimating lens 6 and is transformed into a first luminous flux of approximately parallel light beams. The first luminous flux transmits to the optical path changer 7, which changes the transmitting direction toward the first optical disk. Accordingly, the first luminous flux illuminates the wavelength selector 8. The middle and peripheral portions 82, 83 of the wavelength selector 8 block the first luminous flux, so that only a central part of the first luminous flux passes through the wavelength selector 8 and is incident on the objective lens 9. The objective lens 9 converges the incident first luminous flux to form a focused light spot (not shown) on the first optical disk.

After forming the light spot on the first optical disk, the first optical disk reflects the incident beam as a first return beam (not labeled). The first return beam sequentially passes through/from the objective lens 9, the wavelength selector 8, the optical path changer 7, the collimating lens 6, the pentagonal prism 5 and the prism unit 4, and reaches the first holographic lens 3 a. The first holographic lens 2 a deflects the first return beam toward the first photo detector 2 a. Then, the first photo detector 2 a receives the first return beam and generates corresponding electrical signals.

When recording an information signal on and/or reproducing an information signal from the second optical disk, the second light source 1 b emits a second light beam (not labeled) having the wavelength of about 405 nm. The second light beam propagates through the second holographic lens 3 b along an original direction thereof, and enters the second prism 42 through the second incident surface 4200. The second light beam propagates to the second interface 424 of the second prism 42, and passes through the second interface 424 along its original direction because of its short wavelength. Subsequently, the second light beam is output from the third emergent surface 4222. Subsequently, the second light beam transmits into the third prism 43 through the fifth incident surface 4320. In the third prism 3l, the second light beam passes through the third interface 434 along its original direction because of its short wavelength. Subsequently, the second light beam is output from the fifth emergent surface 4322.

After exiting the prism unit 4, the second light beam transmits into the pentagonal prism 54 through the sixth incident surface 50, and propagates to the sixth emergent surface 51 after being reflected by the sixth reflective surfaces 53 and 52. The second light beam is condensed by the collimating lens 6, and transformed into a second luminous flux of parallel light beams. The second luminous flux transmits to the optical path changer 7, which changes the transmitting direction toward the second optical disk. Accordingly, the second luminous flux illuminates the wavelength selector 8. The wavelength selector 8 does not block any of the second luminous flux, so that the second luminous flux completely passes through the wavelength selector 8 and is incident on the objective lens 9. The objective lens 9 converges the second luminous flux to form a focused light spot (not shown) on the second optical disk.

After forming the light spot on the second optical disk, the second optical disk reflects the incident beam as a second return beam (not labeled). The second return beam sequentially passes through/from the objective lens 9, the wavelength selector 8, the optical path changer 7, the collimating lens 6, the pentagonal prism 5 and the prism unit 3, and reaches the second holographic lens 22. The second holographic lens 3 b deflects the second return beam toward the second photo detector 2 b. Then, the second photo detector 2 b receives the second return beam and generates corresponding electrical signals.

When recording an information signal on and/or reproducing an information signal from the third optical disk, the third light source 1 c emits a third light beam (not labeled) having the wavelength of about 650 nm. The third light beam propagates through the third holographic lens 3 c along an original direction thereof, and enters the second prism 42 through the third incident surface 4220. The third light beam is converged by the third incident surface 4220. In the second prism 42, the third light beam is reflected by the third reflective surface 4226 and the second interface 424, and is then output from the third emergent surface 4222. The third light beam then transmits into the third prism 43 through the fourth incident surface 4300. In the third prism 31, the third light beam passes through the third interface 434 along its original direction because of its relatively short wavelength. Subsequently, the third light beam is output from the fifth emergent surface 4322.

After exiting the prism unit 4, the third light beam transmits into the pentagonal prism 54 through the sixth incident surface 50, and propagates to the sixth emergent surface 51 after being reflected by the sixth reflective surfaces 53 and 52. The third light beam is condensed by the collimating lens 6 and transformed into a third luminous flux of approximately parallel light beams. The third luminous flux transmits to the optical path changer 7, which changes the transmitting direction toward the third optical disk. Accordingly, the third luminous flux illuminates the wavelength selector 8. The peripheral portion 83 of wavelength selector 8 blocks the third luminous flux, so that only central and intermediate parts of the third luminous flux pass through the wavelength selector 8 and are incident on the objective lens 9. The objective lens 9 converges the third luminous flux to form a focused light spot (not shown) on the third optical disk.

After forming the light spot on the third optical disk, the third optical disk reflects the incident beam as a third return beam (not labeled). The third return beam sequentially passes through/from the objective lens 9, the wavelength selector 8, the optical path changer 7, the collimating lens 6, the pentagonal prism 5 and the prism unit 3, and reaches the third holographic lens 3 c. The third holographic lens 3 c deflects the third return beam toward the third photo detector 2 c. Then, the third photo detector 2 c receives the third return beam and generates corresponding electrical signals.

Referring to FIG. 4, an optical pickup head 100′ in accordance with the second embodiment of the present invention is illustrated. A significant difference of the optical pickup head 100′ relative to the optical pickup head 100 is that a composite pentagonal prism 5′ is provided, instead of having the pentagonal prism 5 and the collimating lens 6. The pentagonal prism 5′ includes a collimating surface (not labeled) at an emergent surface thereof, for collimating light beams passing therethrough.

Referring to FIGS. 5 and 6, an optical pickup head 100″ in accordance with the third embodiment of the present invention is illustrated. Compared with the first embodiment, the optical pickup head 100″ utilizes a pechan prism 5″ instead of the pentagonal prism 5. The pechan prism 5″ is a composite prism, and comprises a first portion 51″ and a second portion 52″. The first portion 51″ includes a seventh incident surface 510″, and two seventh reflective surfaces 511″, 512″ adjacent opposite ends of the seventh incident surface 510″. The second portion 52′ includes a seventh emergent surface 520″, and two eighth reflective surfaces 521″, 522″. The seventh and eighth reflective surfaces 511″ and 521″ are partially coated with a reflective coating, and are adhered together to integrate the first and second portions 51″ and 52″. The seventh emergent surface 520″ is parallel to the seventh incident surface 510″. Other parameters of the pechan prism 5″ are as follows: width a=4 mm; width b=4.848 mm; angle φ=45°; angle θ=112.5°; and angle β=67.5°.

When an incident light beam perpendicularly enters the pechan prism 5″ via the seventh incident surface 510″, the light beam is reflected by the reflective coating of the seventh reflective surface 511″ and then by the seventh reflective surface 512″. The reflected light beam propagates to an uncoated part of the seventh reflective surface 511″, passes directly through the seventh reflective surface 511″, and enters the second portion 52″. The light beam obliquely illuminates the seventh emergent surface 520″, is reflected by the seventh emergent surface 520″ and then by the eighth reflective surfaces 522″, 521″, and finally perpendicularly exits the pechan prism 5″ via the seventh emergent surface 520″.

The optical pickup heads 100, 100′ and 100″ provide good performance for all three kinds of optical disks. Both (i) the working wavelength of optical elements, such as the second light source 1 b, the collimating lens 6 and the objective lens 9, and (ii) the numerical aperture of the objective lens 9, are directly matched with requirements of the second optical disk. Therefore, when recording an information signal on and/or reproducing an information signal from the second optical disk, the optical pickup heads 100, 100′ and 100″ provide high quality light convergence to the focused light spot. Further, because the first and second aspherical surfaces are respectively formed on the first prism 41 and the second prism 42, aberrations caused by non-matching between the first and third luminous fluxes and the collimating lens 6 and objective lens 9 are corrected. Moreover, only a central part of the objective lens 9 is illuminated by the first and third light beams, so the NA of the objective lens 9 is reduced when focusing the first and third light beams, and corresponds to the small NA required by the first and third optical disks. Therefore, when recording an information signal on and/or reproducing an information signal from the first and third optical disks, the optical pickup heads 100, 100′ and 100″ provide high quality light convergence to the focused light spot.

The optical pickup heads 100, 100′ and 100″ also have structural and other advantages. Because the first, second and third light beams are reflected by the prism unit 4 and the pentagonal prism 5 (or 5′) or the pechan prism 5″, the optical path length between the optical path changer 7 and the first, second and third light sources 1 a, 1 b and 1 c is reduced. This enables the optical pickup heads 100, 100′ and 100″ to be miniaturized. In addition, the first and second aspherical surfaces are directly formed on the first and second prisms 41 and 42 respectively, so that no extra optical element need be added to the optical pickup heads 100, 100′ and 100″. This further facilitates miniaturization of the optical pickup heads 100, 100′ and 100″, and improves the efficiency of mass production. Furthermore, the collimating lens 6 can be integrally formed with the pentagonal prism 5 (or 5′) or the pechan prism 5″, thereby even further facilitating miniaturization.

Although the present invention has been described with reference to specific embodiments, it should be noted that the described embodiments are not necessarily exclusive, and that various changes and modifications may be made to the described embodiments without departing from the scope of the invention as defined by the appended claims.